This invention relates in general to generator lubrication systems, and more particularly, to aircraft generator, failure mode, cooling-oil lubricated operations for generator units, during an extended interim without power output from a failed generator unit, such as when its primary cooling and lubricating components have ceased to function.
Aviation safety and dependability for profitable operations necessitates redundancy of electrical power generators, as well as, delayed replacement of same so that electrical power continues to be supplied to all vital functions without interruption during relevant operating periods. This means that in a multi-engined aircraft, each engine can be used to power a generator system with each generator system being capable of fulfilling the entire electrical power output needs for the aircraft. This is often carried one step further in safety and redundancy by providing an auxiliary engine and generator which may be used mainly for meeting the aircraft's electrical power needs if there is an emergency with no other remaining back-up propulsion power paired operative generator system.
Aircraft generator lubrication systems have heretofore comprised multiple scavenging pump sections which circulate a fluid lubricant through casings and return a predetermined quantity of the fluid lubricant to a central reservoir which serves combined drive and generator units in an integrated fashion. Intermediate filtering and cooling operations are performed on the cooling-oil lubricant in order to siphon-off contaminants and provide for a stable operating temperature while the generator unit is providing the operating load requirements for output electrical power.
The driven generator system is thus mainly dependent on the cooling-oil fluid for these functions during the operative power producing state of an integrated unit such as in the patent entitled "Cooling Arrangement for and Integrated Drive-Generator System" which is U.S. Pat. No. 4,284,913 and is assigned to the same assignee as the present application. This type of generator system utilizes a constant speed drive transmission which interfaces with the propulsion engine. Its output shaft provides a constant speed so that constant frequency electrical power is provided by the generator system. One of the problems associated with this type of generator system is the excess weight that is introduced by the constant speed drive unit. However, it is possible to eliminate a substantial portion of the weight of the drive unit by more recent innovations in the art of variable speed generator systems.
A type of aircraft electrical power generation system to which this invention also pertains has found widespread acceptance because of its reduction in relative weight compared to generators of the constant speed type and because of the presence of state of the art electronic features. The variable speed constant frequency (VSCF) system according to U.S. Pat. No. 4,554,501, entitled "Variable Speed Constant Frequency Power System With Auxiliary DC Output" is assigned to the same assignee as the present invention, and it is hereby incorporated by reference into the present application and will be hereinafter referred to as the incorporated U.S. patent. This invention takes advantage of the inherent capabilities of a DC-link VSCF system to provide backup auxiliary DC power capability with little weight penalty.
One of the principle adjuncts with generating systems of the latter type is that it does not usually incorporate apparatus for mechanically disconnecting the rotating generator from the propulsion engine coupling when the generator has a pump failure or when there is a loss of cooling-oil fluid which renders the generator incapable of producing electrical power output. Another variable speed generator is of the type described in U.S. Pat. No. 4,851,723 entitled "Coolant Pump System for Variable Speed Generators" which is also assigned to the present assignee, and would also provide a proper environment for the utilization of the presently intended lubrication system.
A faulty generator would continue to rotate until it is disengaged which is usually until either the shear section for the rotating shaft fails or until it is feasible for the maintenance crew to remove the generator from the airplane engine, which action would also suggest that it be replaced with an operative spare unit which calls for making the same available at the right time. This requires that for any degree of aircraft system availability, the number of spare generator units or line replaceable units must be increased to assure that a spare is available when needed.
When a constant speed drive and an associated generator is used, the number of spares needed for the constant speed drive is roughly double that for the number of generator spares required which remains the same. This is because specialized maintenance tools are required to service the constant speed drive, and this results in maintenance time for the constant speed drive that is roughly double that for the VSCF generator.
Another problem with a failed generator is that nearly immediate removal causes a severe impact upon schedules and dependability of commercial service. Unless means are provided for some lubrication to be applied to the bearings, the faulty generator must be removed within three hours after a malfunction of the lubrication system, otherwise permanent damage is anticipated. There is no requirement which prevents the commercial operator from flying an aircraft with a faulty generator as long as there are backups aboard the flyable system, and there is also the incentive for the operator to wait until the aircraft is present at an optimum repair facility before removal of the faulty generator, thereby operating with a minimal impact. Every time there is a pump failure or loss of the cooling-oil fluid by breakage of an external oil line in an electrical power generator, the complication to the air travel system is potentially immediate and may have consequential affects in both the short run and the long run.